1. Field of the Invention
The present invention relates to an image formation apparatus for forming an electrostatic latent image on a photosensitive body by exposing the photosensitive body to a scanned laser beam, and forming an image corresponding to the laser beam by developing the electrostatic latent image.
2. Description of the Related Art
As an image formation apparatus using an electrophotographic process, a laser printer is generally used. The laser printer includes a laser beam scanning unit for emitting a laser beam according to an image signal, a photosensitive drum on which an electrostatic latent image is formed by the emission of a laser beam, and processing members for applying processes of electrifying, developing and cleaning the photosensitive drum. In this laser printer, image formation is made by emitting a laser beam according to an image signal; exposing the uniformly electrified photosensitive drum to the scanned laser beam for forming an electrostatic latent image according to the image signal, and sticking toner or the like on the photosensitive drum thus forming a toner image. The toner image thus formed on the photosensitive drum is transferred and fixed on a recording paper sheet or the like.
In general, such a laser beam printer includes a safety interlock. When a cover of the printer is opened, the safety interlock acts in co-operation with the cover to shield a space between a photosensitive drum and the opening of the cover by means of a shutter, or to stop the operation of a laser beam scanning unit. During maintenance or the like of the printer, the safety interlock prevents an operator from being exposed to the laser beam leaked from the printer.
As laser beam printers have been significantly spread in recent years, there have been strong demands toward miniaturization. To meet these demands, attempts have been made to substantially simplify the internal structure of the printer. Specifically, there have been commercially available laser beam printers of a type using a cartridge (EP cartridge) integrally containing a developing unit for containing toner; cleaning unit and electrifying unit for removing or imparting electrostatic charge thereby preparing a state where an electrostatic latent image can be formed; and a photosensitive drum. In this laser beam printer, the main maintenance can be performed only by removing the EP cartridge, thus improving the operability of the printer.
Even in the laser beam printer simplified in the internal structure, each of functional parts must be mounted with a high accuracy for obtaining an image with a high quality. To cope with this requirement, in some printer, a frame to which each of functional parts is mounted is made of a resin having a high rigidity such as polycarbonate containing glass fiber in an amount of from 30 to 50 wt %.
Moreover, to mount the EP cartridge at an accurate position, a cover which is opened/closed upon mounting/dismounting the EP cartridge, is also increased in its strength for pressing the EP cartridge mounted in the frame.
However, in the case where glass fiber is contained in a resin for increasing the rigidity of a frame and a cover made of the resin, there occurs a problem in damaging the surface of a die during molding. This increases the wear of a die for molding a frame of a laser beam printer, and also causes an inconvenience in the quality of the appearance of the finished frame and thereby the frame is required to be covered with a casing thus obstructing the reduction in the manufacturing cost.
Moreover, a resin containing glass fiber is increased in weight more than a resin with no glass fiber (for example, the addition of glass fiber in polycarbonate in an amount of 30 wt % increases the weight by about 1.43 times). This makes it difficult to reduce the weight of a printer reduced in size.